FIG. 1 illustrates one type of prior art color, transmissive LCD.
In FIG. 1, an LCD 10 includes an LED white light source 12 to provide backlighting for the upper LCD layers. The LED white light source 12 has certain advantages over using a traditional fluorescent bulb, such as size, reliability, and avoiding the use of a high voltage power supply.
For other than very small displays, there are typically multiple white light LEDs used in a backlight to more uniformly distribute the light to the bottom of the LCD layers and supply the required brightness level. For small and medium LCD backlights, the white light LEDs may be optically coupled to one or more edges of a transparent lightguide that uniformly leaks light out its top surface. Light from multiple LEDs somewhat mixes within the lightguide. For medium and large LCD backlights, an array of the white light LEDs may be positioned on the bottom surface of a light mixing box. A diffuser, a brightness enhancement film (BEF), and a dual brightness enhancement film (DBEF) are positioned over the lightguide or box opening to smooth and direct the light for illuminating the LCD layers. Designers have strived to make all of the backlight white light LEDs output the same target white point so that the backlight outputs a uniform white point across the surface of the backlight.
The layer 13 in FIG. 1 represents the edge-lit lightguide and/or any mixing optics (mixing box, diffuser, BEF, DBEF, etc.) used for the particular application. The combination of the light source 12 and lightguide/mixing optics (layer 13) is referred to as a backlight 14.
A polarizing filter 15 linearly polarizes the white light. The polarized white light is then transmitted to a transparent thin film transistor (TFT) array 16, having one transistor for each red, green, and blue subpixel in the display. A set of closely spaced red, green, and blue subpixels is referred to as a white pixel whose color “dot” is a combination of the three subpixels. If the RGB subpixels are all energized, the dot creates white light. TFT arrays are well known. The TFT array is controlled by an LCD controller 17.
Above the TFT array 16 is a liquid crystal layer 20, and above liquid crystal layer 20 is a transparent conductive layer 22 connected to ground. In one type of LCD, an electrical field across a subpixel area of the liquid crystal layer 20 causes light passing through that subpixel area to have its polarization rotated orthogonal to the incoming polarization. The absence of an electrical field across a subpixel area of the liquid crystal layer 20 causes the liquid crystals to align and not affect the polarization of light. Selectively energizing the transistors controls the local electric fields across the liquid crystal layer 20. Each portion of the liquid crystal layer associated with a subpixel is commonly referred to as a shutter, since each shutter is controllable to pass from 0-100% (assuming a lossless system) of the incoming light to the output of the display. Liquid crystal layers are well known and commercially available.
A polarizing filter 24 only passes polarized light orthogonal to the light output from the polarizing filter 15. Therefore, the polarizing filter 24 only passes light that has been polarized by an energized subpixel area in the liquid crystal layer 20 and absorbs all light that passes through non-energized portions of the liquid crystal layer 20. The magnitudes of the electric fields across the liquid crystal layer 20 control the brightness of the individual R, G, and B components to create any color for each pixel in the displayed image.
Other types of LCDs pass light through only the non-energized pixels. Other LCDs use different orientation polarizers. Some types of LCDs substitute a passive conductor grid for the TFT array 16, where energizing a particular row conductor and column conductor energizes a pixel area of the liquid crystal layer at the cross-point.
The light passing through the polarizing filter 24 is then filtered by an RGB pixel filter 25. The RGB pixel filter 25 can be located at other positions in the stack, such as anywhere below or above the liquid crystal layer 20. The RGB pixel filter 25 may be comprised of a red filter layer, a green filter layer, and a blue filter layer. The layers may be deposited as thin films. As an example, the red filter layer contains an array of red light filter areas defining the red subpixel areas of the display. Similarly, the green and blue filter layers only allow green or blue light to pass in the areas of the green and blue subpixels. Accordingly, the RGB pixel filter 25 provides a filter for each R, G, and B subpixel in the display.
The RGB pixel filter 25 inherently filters out at least two-thirds of all light reaching it, since each filter subpixel area only allows one of the three primary colors to pass. This is a significant factor in the generally poor efficiency of the prior art LCDs. The overall transmissivity of the LCD layers above the backlight 14 is on the order of 4-10%.
One type of white light LED 30 is shown in FIG. 2. The LED 30 comprises a flip-chip LED die formed of a semiconductor light emitting active layer 32 between a p-type layer 33 and an n-type layer 34. Optionally, the growth substrate (e.g., sapphire) is removed. The LED die emits blue light. Typical materials for the LED die are GaN and SiC. Examples of forming such LEDs are described in U.S. Pat. Nos. 6,649,440 and 6,274,399, both assigned to Philips Lumileds Lighting and incorporated by reference.
The LED die is mounted on a submount 36 formed of any suitable material, such as a ceramic or silicon. The LED die has bottom metal contacts 38 that are bonded to associated metal pads 40 on submount 36 via gold balls 44. Vias through submount 36 terminate in metal pads on the bottom surface of submount 36, which are bonded to metal leads on a circuit board 46. The metal leads are connected to other LEDs or to a power supply.
Since the LED die only emits blue light, red and green light components must be added to create white light. Such red and green components are provided by a phosphor layer 48 that contains red and green phosphors or contains a yellow-green phosphor (e.g., YAG). The phosphor layer 48 may also cover the sides of the LED die. There are many known ways of providing a phosphor layer over a blue die to create white light.
The phosphor layer 48 allows a percentage of the blue LED light to leak out. Some of the blue light is absorbed by the phosphor and reemitted as red and green light (or yellow-green light). The combination of the blue light and phosphor emission creates white light. A target white point is achieved by selecting the densities of the phosphor particles in the layer, the relative amounts of the phosphors, and the thickness of the phosphor layer.
Even though blue LEDs from a manufacturer may be fabricated using the same repeated procedures, the dominant wavelengths of blue LEDs vary from batch to batch and even within a single batch. When the dominant wavelength is important for particular applications, manufacturers energize the blue LEDs and measure their dominant wavelengths, then bin the LEDs according to their dominant wavelengths. The dominant wavelengths may differ by 40 nm, and each bin may typically include LEDs within a 5-8 nm range (i.e., 2.5-4 nm variation from the bin's center wavelength). A typical range of dominant wavelengths for blue LEDs used as LCD backlights is 420-460 nm.
Heretofore, designers of backlights have strived to make the white points of all backlight LEDs the same, so that the human eye will perceive the same white light being emanated across the backlight surface and from one backlight to another. This may be done by precisely matching the blue LED dies and precisely replicating the phosphor layer characteristics for each die. This is wasteful of those LED dies that do not match the target dominant wavelength. Alternatively, the phosphor layer characteristics can be tailored for each blue LED bin so that the resulting white point for each LED matches a single target white point. All such white light will appear identical to the human eye.
The present inventors have measured the light attenuation by LCD layers versus wavelength and have determined that the attenuation varies with wavelengths within the visible range. The variation within the blue wavelength range is by far the greatest. The attenuation is due to the combined attenuation by the polarizers, the ITO electrode (the transparent grounded layer), the liquid crystal layer, and the RGB filters. There is also non-flat attenuation by the lightguide (if used), the diffuser (if used), and BEFs (if used).
The inventors have discovered that, because of the varying light attenuation by LCD layers versus wavelength, even though the white points of the backlight LEDs are matched by tailoring the phosphor characteristics for each bin of blue LEDs, the measured color output by the LCD is not consistent across the surface of the LCD, such as when all pixels are turned fully on to create a solid white light display.
Therefore, what is needed is an LED backlight that results in the color output of an LCD to be consistent across the surface of the LCD and from one LCD to another.